Identification of Plasmopara viticola genes potentially involved in pathogenesis on grapevine suggests new similarities between oomycetes and true fungi.

Plant diseases caused by fungi and oomycetes result in significant economic losses every year. Although phylogenetically distant, these organisms share many common features during infection. We identified genes in the oomycete Plasmopara viticola that are potentially involved in pathogenesis in grapevine by using fungal databases and degenerate primers. Fragments of P. viticola genes encoding NADH-ubiquinone oxidoreductase (PvNuo), laccase (PvLac), and invertase (PvInv) were obtained. PvNuo was overexpressed at 2 days postinoculation (dpi), during the development of the first hyphal structures and haustoria. PvLac was overexpressed at 5 dpi when genes related to pterostilbene biosynthesis were induced in grapevine. Transcript level for PvInv increased between 1 and 4 dpi before reaching a plateau. These results might suggest a finely tuned strategy of infection depending on nutrition and plant response. Phylogenetic analyses of PvNuo showed that P. viticola clustered with other oomycetes and was associated with brown algae and diatoms, forming a typical Straminipila clade. Based on the comparison of available sequences for laccases and invertases, the group formed by P. viticola and other oomycetes tended to be more closely related to Opisthokonta than to Straminipila. Convergent evolution or horizontal gene transfer could explain the presence of fungus-like genes in P. viticola.

A beta-1,3 glucan sulfate induces resistance in grapevine against Plasmopara viticola through priming of defense responses, including HR-like cell death.

Sulfated laminarin (PS3) has been shown previously to be an elicitor of plant defense reactions in tobacco and Arabidopsis and to induce protection against tobacco mosaic virus. Here, we have demonstrated the efficiency of PS3 in protecting a susceptible grapevine cultivar (Vitis vinifera cv. Marselan) against downy mildew (Plasmopara viticola) under glasshouse conditions. This induced resistance was associated with potentiated H2O2 production at the infection sites, upregulation of defense-related genes, callose and phenol depositions, and hypersensitive response-like cell death. Interestingly, similar responses were observed following P. viticola inoculation in a tolerant grapevine hybrid cultivar (Solaris). A pharmacological approach led us to conclude that both callose synthesis and jasmonic acid pathway contribute to PS3-induced resistance.